Epoxy resins are well known for use in making advanced or high performance composites comprising high strength fiber made of glass, boron, carbon or the like. Structures made of these composites can weigh considerably less than their metal counterparts at equivalent strength and stiffness. Higher modulus epoxy composites, however, have been relatively brittle. This brittleness has restricted their wider application in the aerospace field because, for example, damage tolerance, an important property of flight critical components in aircraft, is related to brittleness of the component. In addition, brittleness is generally believed to be associated with lower peel strength values of high Tg epoxy thermosets used as structural adhesives.
One approach in making epoxy thermosets tougher and have higher peel strengths has been to introduce reactive rubbery polymers into the epoxy resin formulations. See Diamant and Molton, "Development of Resins For Damage Tolerant Composites--A Systematic Approach," 29th National SAMPE Symposium, Apr. 3-5, 1984. See, also, R. S. Drake and A. R. Siebert, SAMPE Quarterly, 6(4) (1975) and R. S. Drake and A. R. Siebert, Organic Coatings and Applied Polymer Science Division Proceedings, 48, 491 (1983).
Another approach for modifying epoxy thermosets has been to incorporate engineering thermoplastics into the crosslinked epoxy matrix. Various thermoplastics have been suggested and the use of a polyethersulfone as the thermoplastic modifier for epoxy resins was studied by C. B. Bucknall et al and is discussed in the British Polymer Journal, Vol. 15, March 1983 at pages 71 to 75. Bucknall et al's studies were carried out on cured epoxy-polyethersulfone blends prepared from mixtures of trifunctional and tetrafunctional aromatic epoxides, diaminodiphenylsulfone or dicyandiamide as hardener and various amounts of Victrex lOO P manufactured by ICI Ltd., said to be a relatively low molecular weight grade of polyethersulfone. The studies showed that phase separation occurred in certain of the cured epoxy-polyethersulfone blends and that some of the cured blends exhibited distinct nodular morphological features. Analyses indicated that the polyethersulfone was concentrated in the nodules and Bucknall et al surmised that the nodules were not formed by polyethersulfone alone but by a crosslinked epoxy-polyethersulfone copolymer. Bucknall et al found no clear correlation between mechanical properties such as elastic modulus, fracture toughness and creep of the cured blends and composition and concluded that the addition of polyethersulfone has little effect on the fracture toughness of the resin mixtures, irrespective of the degree of phase separation or the morphology.